Technical Field
The present invention relates to a method for producing a tube of glass, particularly of quartz glass, comprising the following method steps:                (a) providing a hollow cylinder of the glass having a wall thickness and having an outer diameter D1,        (b) zonewise heating and softening the hollow cylinder which is rotating about a rotation axis in a heating zone moved relative to the rotation axis,        (c) forming a deformation zone by radial expansion of the softened area under the action of a centrifugal force and/or an internal overpressure applied in the hollow cylinder bore, and        (d) continuously forming the tube to produce an outer diameter D2 which is greater than D1.        
Moreover, the present invention relates to an apparatus for performing such a method, comprising:                a rotation device for rotating a hollow cylinder of glass about its longitudinal axis, which cylinder has an inner diameter, an outer diameter D1 and an inner bore defined by a wall, and        a heater which is movable relative to the hollow cylinder for the zonewise heating and softening of the hollow cylinder and for forming a tube having an outer diameter D2 which is greater than D1.        
With the help of such methods and apparatuses, hollow cylinders of glass, particularly of quartz glass, are formed in one or several hot forming steps into tubes having an increased outer diameter. An initial hollow cylinder which is rotating about its longitudinal axis is here softened zone by zone in a heating zone, which is moved at a relative feed rate in relation to the hollow cylinder, and is expanded in this process under the action of a radially outwardly directed force either against a molding tool arranged at a given radial distance from the longitudinal axis of the tube, or it is formed without tools. The radially outwardly directed force is based on the centrifugal force and/or on an internal overpressure in the inner bore of the hollow cylinder (also called “blow pressure”).
To observe the dimensional accuracy of the drawn-off tube strand, at least one of its dimensions, for example outer diameter, inner diameter, or wall thickness, is controlled. The blow pressure, the relative feed rate between hollow cylinder and heating zone, and the temperature in the heating zone are common manipulated variables of the control.
Prior Art
The larger the tube end diameter, the more difficult and expensive becomes the production of a dimensionally accurate large tube. To mitigate these problems, Japanese patent application publication JP 2004-149325 A suggests that the forming process should be subdivided into a plurality of forming steps with successive increase in diameter. To this end the hollow cylinder of quartz glass to be formed, having a diameter of 250 mm, is clamped in a lathe and rotated about its horizontally oriented longitudinal axis while being heated by an annular arrangement of heating burners and is thereby softened zone by zone, wherein the heating burners are moved at a given feed rate along the cylinder jacket. The increase in diameter is due to the centrifugal force acting on the softened region. The deformation zone will travel once along the entire initial cylinder until the cylinder is completely expanded. The outer diameter of the tube is here sensed continuously without tools by a laser beam. This forming step will be repeated until the nominal tube diameter of 440 mm is reached. In each forming step the tube diameter is increased by 15 mm.
In this forming process, one achieves a comparatively small forming degree in each individual forming step, which is accompanied by a reduced deviation from the target value of a radial tube dimension. Moreover, in each forming step it is possible to take into account and correct dimensional deviations existing in the respective initial cylinder.
On the other hand, it is evident that this procedure is very time- and energy-consuming, especially since the tube cools down between successive forming steps.
European patent application publication EP 0 037 648 A1 describes a method of producing optical fibers in which a tube is formed by zonewise heating and application of an internal overpressure into a tube having an increased inner diameter.
U.S. Pat. No. 5,167,420 describes an apparatus for producing a surrounding groove in a glass tube, wherein the viscosity of the glass is reduced in the area of the groove by active cooling.
Japanese patent application publication JP H10-101353 A describes a method for producing a quartz glass tube, wherein a quartz glass cylinder is softened zone by zone by applying an internal overpressure and is formed while rotating about its longitudinal axis against an outer molding tool into the tube. The quartz glass cylinder is here closed at one side. Besides the outer diameter, it is the aim to achieve a uniform wall thickness. To this end parallel mold plates are used on the molding tool.
German Patent DE 41 21 611 C1 describes a method for producing quartz glass tubes, in which the wall thickness of the drawn-off quartz-glass tube strand is regulated. A hollow cylinder of quartz glass is here pushed continuously while rotating through a heating furnace within which water-cooled graphite plates are arranged at a radial distance from the longitudinal axis of the tube. Due to overpressure within the hollow cylinder the soft hollow cylinder is blown against the graphite plates, so that the radial distance of the graphite plates from the longitudinal axis of the tube roughly predetermines the resulting outer diameter of the tube. Viewed in the feed direction of the blank relative to the furnace, soft quartz glass accumulates in front of the graphite plates and forms a circumferential bead around the outer wall of the blank. It is suggested that the height of the circumferential bead should be used for process control by optically sensing the bead height by a camera and by using the deviation from a predetermined target bead height for process control. The overpressure in the inner bore of the hollow cylinder is chosen as a manipulated variable of the control. Variations of the inner diameter of the tube and thus variations of the wall thickness of the tube can thereby be minimized.
Technical Object
It may be tried to keep the number of forming steps as small as possible, wherein the respective deformation degree, i.e. the change in diameter, is set to be as high as possible. However, it has been found that dimensional deviations already existing in the original hollow cylinder tend to continue into the drawn-off glass tube in the forming process and are even intensified. Variations in the radial cross-sectional profile or wall one-sidedness, i.e. radially irregular course of the tube wall thickness, which is also called “siding” among the experts, are particularly disadvantageously noticeable. Since the outer diameter is a relatively fixedly predetermined value in the use of a molding tool, tube wall siding is in this case accompanied by fluctuations in the inner diameter of the tube.
With increasing tube end diameter, these problems increase. The reason is that in the forming process wall thickness variations, which are found in the initial cylinder, exponentially rise with the diameter. Therefore, the maximum values for siding (e.g. 1 mm), which are still tolerable according to the specification, may in the final analysis limit the tube end diameter that can be realized in practice. Comparatively thin wall areas of the hollow cylinder deform more easily than rather thick-walled areas. The greater the blow pressure, the more the thickness difference will be noticed, so that the blow pressure cannot be arbitrarily high. Instead, in order to achieve commercially acceptable forming rates, the glass must be heated at a higher temperature and softened more strongly. This, however, results in pronounced drawing streaks and other defects in the glass wall and in an increased energy demand, especially in the case of large-volume tubes (hereinafter also called “large tubes”), which on account of their large size cool down particularly rapidly.